Received 14 April 2004; revised 14 April 2004; accepted
12 September 2004. Available online 16 November 2004.

Abstract

In 1988, radiocarbon laboratories at Arizona, Cambridge, and Zurich
determined the age of a sample from the Shroud of Turin. They reported that the
date of the cloth's production lay between A.D. 1260
and 1390 with 95% confidence. This came as a surprise in view of the technology
used to produce the cloth, its chemical composition, and the lack of vanillin in
its lignin. The results prompted questions about the validity of the sample.

Preliminary estimates of the kinetics constants for the loss of vanillin from
lignin indicate a much older age for the cloth than the radiocarbon analyses.
The radiocarbon sampling area is uniquely coated with a yellow–brown plant gum
containing dye lakes. Pyrolysis-mass-spectrometry results from the sample area
coupled with microscopic and microchemical observations prove that the
radiocarbon sample was not part of the original cloth of the Shroud of Turin.
The radiocarbon date was thus not valid for determining the true age of the
shroud.

Article Outline

1. Introduction

The Shroud of Turin is a large piece of linen that shows the faint image of a
man on its surface. Many people believe it is the burial cloth of Jesus, making
it extremely controversial. Radiocarbon ages were determined in 1988 [1],
which should have settled controversies as to the age of the linen. The 1988
radiocarbon age determinations were the best that could have been obtained.
Sample preparation methods were compared and confirmed, and the measurements
were made with the best available instruments. Damon et al. reported [1]
that “The age of the shroud is obtained as A.D.
1260–1390, with at least 95% confidence.” However, that date does not agree with
observations on the linen-production technology nor the chemistry of fibers
obtained directly from the main part of the cloth in 1978 [2]
and [3].
The 1988 sampling operation was described in [1]:
“The shroud was separated from the backing cloth along its bottom left-hand edge
and a strip (10 mm × 70 mm) was cut from just above the place
where a sample was previously removed in 1973 for examination. The strip came
from a single site on the main body of the shroud away from any patches or
charred areas.” Franco Testore, professor of textile technology at the Turin
Polytechnic, and Gabriel Vial, curator of the Ancient Textile Museum, Lyon,
France, approved the location of the radiocarbon sample. However, the operation
was done in secrecy, and no chemical investigations were made at the time to
characterize the sample.

2. Samples

Professor Gilbert Raes of the Ghent Institute of Textile Technology cut the
1973 sample [4]
mentioned by Damon et al. [1].
Raes found that one part of his sample contained cotton, but the part on the
other side of a seam did not. He reported that the cotton was an ancient Near
Eastern variety, Gossypium herbaceum, on the basis of the distance between
reversals in the tape-shaped fibers (about eight per centimeter). I received 14
yarn segments from the Raes sample from Prof. Luigi Gonella (Department of
Physics, Turin Polytechnic University) on 14 October 1979. I photographed the
samples as received and archived them separately in numbered vials. Some of the
samples were destroyed in chemical tests between 1979 and 1982, but most of the
segments have been preserved.

As part of the shroud of turin research project (STURP), I took 32
adhesive-tape samples from all areas of the shroud and associated textiles in
1978 [2].
Ronald Youngquist of the Minnesota Mining and Manufacturing Corporation produced
the tape specifically for the project; an amorphous, inert, pure-hydrocarbon
adhesive that would not contaminate the shroud or the samples. It enabled direct
chemical testing on recovered linen fibers and particulates, and the adhesive
could be removed by washing with xylene. I applied the tapes to the surface of
the shroud with a pressure-measuring applicator to enable semi-quantitative
comparisons among samples.

The shroud was badly damaged in a church fire in A.D. 1532. Nuns patched burn holes and stitched the shroud
to a reinforcing cloth that is now known as the Holland cloth. I also sampled it
in 1978. The Holland cloth provides an authentic, documented sample of linen as
it was produced in Europe between 1532 and 1534.

On 12 December 2003, I received samples of both warp and weft threads that
Prof. Luigi Gonella had taken from the radiocarbon sample before it was
distributed for dating. Gonella reported that he excised the threads from the
center of the radiocarbon sample.

3. Alternate methods for linen age estimation

STURP was not allowed to take radiocarbon samples in 1978; therefore, it was
useful to devise independent methods for age determination to test the validity
of the published date [1].
The most obvious aging change is the deepening sepia color of linen. Absolute
spectral reflectance measurements were made on the shroud and many museum
samples of linen by Gilbert and Gilbert [5].
No simple relationship between color and age could be derived, because bleaching
methods have changed through the centuries.

The crystalline structure of flax fibers acts like a dosimeter: it collects
radiation defects over time. Unfortunately, background radiation varies greatly
with storage conditions and latitude. Radon collects in many old buildings and
tombs, and alpha-particles cause short, intensely birefringent ionization
tracks. Electromagnetic radiation (ultraviolet, X-ray, and gamma-ray) causes a
diffuse background birefringence that varies with location and linen-production
methods. Neutrons produce recoil protons in the interior of fibers, and
energetic protons produce long ion tracks that occasionally branch.
Radiation-based age estimation is very subjective; however, defect populations
indicate that the shroud's linen has been absorbing several kinds of radiation
for a very long time.

The lignin at growth nodes on the shroud's flax fibers (Fig.
1) did not give the usual chemical spot test for lignin (i.e., the
phloroglucinol/HCl test for vanillin). The Holland cloth and other medieval
linens gave a clear test. This suggested that the rate of loss of vanillin from
lignin could offer a method for estimating the age of the shroud. The
phloroglucinol–hydrochloric-acid reagent detects vanillin
(4-hydroxy-2-methoxybenzaldehyde) with good sensitivity. The lignin on shroud
samples and on samples from the Dead Sea scrolls does not give the test. If the
detection limit for the test is assumed to be constant, aged samples of lignin
can be compared by observing the times and temperatures that give the last
observable color reactions. Stanley T. Kosiewicz of Los Alamos aged samples that
contained lignin at 40, 70, and 100 °C for up to 24 months. Comparison of
detection limits among the samples showed the rate of vanillin loss is very low.
A suitable chemical-age predictive model therefore could be produced.

(72K)

Fig. 1. Colored image fibers from the back of the ankle (400×) that
are still embedded in the sampling tape's adhesive. Dark lignin deposits are
easily visible at the growth nodes. The deposits do not give the spot test for
lignin.

Rates of all kinds of chemical reactions are modeled with the Arrhenius
expression

(1)

where
α is the fraction reacted at any time t; k is the rate
constant, s−1; Z is the Arrhenius frequency factor,
s−1; E is the Arrhenius activation energy,
J mol−1; R is the gas constant,
8.314 J K−1 mol−1; and T is the
absolute temperature in kelvins. f(α) is the depletion factor; and
it depends on the physical state of the reactant, the type of reaction, and/or
the number of molecules involved in the reaction. Analysis of the
time/temperature/detection-limit data gave the following Arrhenius predictive
model for the rate of vanillin loss from lignin.

(2)

After the
rate constant has been calculated at a specific temperature, the amount of time
required for vanillin to be depleted by a first-order reaction at that
temperature can be calculated from

The major problem in estimating the age of the shroud is the fact that the
rate law is exponential; i.e., the maximum diurnal temperature is much more
important than is the lowest storage temperature. However, some reasonable
storage temperatures can be considered to give a range of predicted ages. If the
shroud had been stored at a constant 25 °C, it would have taken about 1319
years to lose a conservative 95% of its vanillin. At 23 °C, it would have
taken about 1845 years. At 20 °C, it would take about 3095 years.

If the shroud had been produced between A.D. 1260
and 1390, as indicated by the radiocarbon analyses, lignin should be easy to
detect. A linen produced in A.D. 1260 would have
retained about 37% of its vanillin in 1978. The Raes threads, the Holland cloth,
and all other medieval linens gave the test for vanillin wherever lignin could
be observed on growth nodes. The disappearance of all traces of vanillin from
the lignin in the shroud indicates a much older age than the radiocarbon
laboratories reported.

The fire of 1532 could not have greatly affected the vanillin content of
lignin in all parts of the shroud equally. The thermal conductivity of linen is
very low,
2.1 × 10−4 cal cm−1 s−1 °C−1;
therefore, the unscorched parts of the folded cloth could not have become very
hot. The temperature gradient through the cloth in the reliquary should have
been very steep, and the cloth's center would not have heated at all in the time
available. The rapid change in color from black to white at the margins of the
scorches illustrates this fact.

Any heating at the time of the fire would decrease the amount of vanillin in
the lignin as a function of the temperature and time heated; however, different
amounts of vanillin would have been lost in different areas. No samples from any
location on the shroud gave the vanillin test.

Because the shroud and other very old linens do not give the vanillin test,
the cloth must be quite old. It is thus very unlikely that the linen was
produced during medieval times.

All threads from the Raes sample and the yarn segments from the radiocarbon
sample show colored encrustations (or coatings) on their surfaces (Fig.
2 and Fig.
3). The coating material is not removed by nonpolar solvents, but it swells
and dissolves in water. There was absolutely no coating with these
characteristics on either the Holland cloth or the main part of the shroud. When
Raes and radiocarbon-sample threads were teased open at both ends with a
dissecting needle, the cores appeared to be colorless, suggesting the color and
its vehicle were added by wiping a viscous liquid on the outside of the yarn. A
marked difference between inside and outside fibers is characteristic of both
the Raes and radiocarbon samples. The yellow–brown coating on the outside of the
radiocarbon warp sample is so heavy that it looks black by transmitted light (Fig.
2). Chemical tests on both the radiocarbon and Raes samples show their
coatings to consist of a plant gum containing alizarin dye present in two forms.
Some is dissolved in the gum, giving it a yellow color. A variable amount is
complexed with hydrous aluminum oxide [AlO(OH)] to form red lakes (Fig.
3). The lakes are gelatinous and usually very small. A good microscope is
required to observe them, and the gum vehicle for the dye/mordant system on the
Raes and radiocarbon samples makes it difficult to observe the lakes. The gum
cannot be removed without damaging the lakes, but it can be made invisible by
matching its index of refraction in a 1.515 index oil. With the gum invisible or
swelled slightly in water, it is easy to see the lakes suspended in the gum and
stuck to the fibers. Fig.
3 shows (upper left) colloidal red dye lakes suspended in the gum. To the
right of that, some discrete lakes can be seen adhering to the surface of a
cotton fiber. Several areas of yellow-dyed gum can be seen. Four cotton fibers
and two flax fibers appear in the view. The radiocarbon sample contains both a
gum/dye/mordant coating and cotton fibers. The main part of the shroud does not
contain these materials.

(55K)

Fig. 2. Fibers from the surface of a radiocarbon warp segment, dry at
100×.

(62K)

Fig. 3. Warp fibers from the radiocarbon sample, 800× in water. The
gum is swelling, becoming more transparent, and detaching from the fibers.

Alizarin and purpurin are extracted from Madder root and first appeared in
Italy about A.D. 1291 [6].
Alizarin has long been used as an acid–base indicator in chemical analysis. It
is yellow below pH 5.6, red above pH 7.2, and purple above pH 11.0. The colored
coating shows all of these changes as a function of pH. HCl (6N) brings the
lakes into solution and turns bright yellow. Alum has been a common mordant for
millennia. The red lakes are diagnostic for Madder root dyes and alum. The
solubility characteristics of the red lakes indicate AlO(OH). The coating was
insoluble at pH 8.0 but dissolves at both lower and higher pH. The red
dye/mordant lakes dissolved in 2N NaOH to give a purple solution. The presence
of aluminum in the coating material is consistent with the results of Adler,
Selzer, and DeBlase [7],
who performed X-ray elemental analyses on different shroud materials, including
fibers from radiocarbon-sample warp threads. They reported concentrations of
aluminum on the radiocarbon sample 20-times those on shroud fibers. Mordants
other than AlO(OH) produce different colors with Madder root dye. Calcium
compounds produce blue colors, and a few blue lakes can be seen on some
gum-coated fibers. They are removed with 6N HCl. The color suggests alizarin on
crystals of calcite or aragonite in the threads.

The presence of alizarin dye and red lakes in the Raes and radiocarbon
samples indicates that the color has been manipulated. Specifically, the color
and distribution of the coating implies that repairs were made at an unknown
time with foreign linen dyed to match the older original material. Such repairs
were suggested by Benford and Marino [8]
and [9].
The consequence of this conclusion is that the radiocarbon sample was not
representative of the original cloth.

The gum coating was quickly hydrolyzed by either concentrated HCl or 2N NaOH.
That fact and its solubility in water suggest that it is probably a
polysaccharide and not a denatured protein. Hydrolysis at a moderate rate in 6N
HCl suggests that it is probably a pentosan, composed of five-carbon sugar
units. Pentosan plant gums form a reversible, bright-yellow color with aqueous
iodine. The color completely disappears after the solution evaporates, proving
that the color is not a result of iodination reactions or iodine-catalyzed
dehydrations. Both Raes and radiocarbon samples give this reaction.

One of the analytical methods used during the STURP studies was pyrolysis
mass spectrometry. The Midwest Center for Mass Spectrometry (MCMS) at the
University of Nebraska, Lincoln, made dozens of scans on different samples in
1981. The chemical-ionization system used was the most sensitive MS at the time,
sufficiently sensitive to detect parts-per-billion traces of oligomers from the
polyethylene bag that Gonella had used to wrap the Raes threads. The instrument
at MCMS is equipped with a pulsed source that has a time resolution of 100 ns,
and it produces a series of mass spectra as the sample heats up. However, it was
impossible to quote an accurate, absolute sample temperature when single
microfibers were being analyzed, only relative sample temperatures could be
compared. Seven different samples underwent multiple analyses at MCMS. Results
were corrected with instrument blanks. Perfluorokerosene was used as an internal
calibrant, its spectrum was subtracted by the instrument's computer. Total ion
currents (TIC) were measured.

Some of the samples came from areas of apparent blood flows, some from
scorched areas, one (“the Zina thread”) was a complete yarn segment that had
been withdrawn from the heel image area, one came from a pure image area, one
came from a water stain in an image area, and several were modern reproductions
of ancient linen technology. Mrs. Kate Edgerton (deceased), made the
reproductions from raw flax according to historic accounts of ancient
technology. To obtain replicate data, some of the pyrolysis/ms analyses had to
be run on single 10–15-μm-diameter fibers that were 5–6 mm long. Compared
with fibers extracted from the sampling tapes, there was ample material from the
Raes sample, which should be representative of the entire Raes/radiocarbon
sampling area.

At the time the samples were analyzed, the goal of STURP was to test whether
the image had been painted. The results of the pyrolysis/ms analyses proved to
be consistent with an array of independent observations and data [2],
showing that the image was not the result of an applied material. With that
conclusion established, the dozens of pyrolysis/ms data sets can now be
reanalyzed to compare samples from the radiocarbon area with samples from the
main cloth.

Cellulose pyrolyzes to produce hydroxymethylfurfural (mass 126), which begins
to deformylate in a series reaction to produce furfural (mass 96). Furfural is
always a minor product of cellulose decomposition. Linen fibers from the main
part of the shroud did not show significant product evolution until relatively
high temperatures (probably about 260 °C), but the products contained both
expected fragments (Fig.
4). Pentosans do not produce hydroxymethylfurfural. They also pyrolyze much
faster at lower temperatures than cellulose. When the first pyrolysis products
appeared during heating, the Raes fibers showed a signal for furfural at mass 96
(Fig.
5). There was no signal at mass 126. These results prove that the gum
coating on the Raes and radiocarbon samples is a pentosan. None can be detected
on any fibers from the main part of the shroud.

(9K)

Fig. 4. A mass spectrum obtained from the pyrolysis of a shroud-image
fiber. The sample did not produce a significant amount of furfural (mass 96)
until it reached higher temperatures; i.e., it did not contain a significant
amount of pentose sugars or pentosans. The ordinate shows the relative ion
intensity for each product at that temperature, and the abscissa shows the
mass of the ion.

(9K)

Fig. 5. A mass spectrum from the low-temperature pyrolysis of fibers
from a Raes sample. As shown by the signal at mass 96 and the absence of a
signal at mass 126, the sample contained a significant amount of pentosan. The
ordinate shows the relative ion intensity for each product at that
temperature, and the abscissa shows the mass of the ion.

The fact that vanillin can not be detected in the lignin on shroud fibers,
Dead Sea scrolls linen, and other very old linens indicates that the shroud is
quite old. A determination of the kinetics of vanillin loss suggests that the
shroud is between 1300- and 3000-years old. Even allowing for errors in the
measurements and assumptions about storage conditions, the cloth is unlikely to
be as young as 840 years.

A gum/dye/mordant coating is easy to observe on Raes and radiocarbon yarns.
No other part of the shroud shows such a coating. The early thermal evolution of
furfural during pyrolysis/ms analyses, the relatively easy water solubility, the
yellow color formed with iodine, and the easy hydrolysis suggest gum Arabic. Gum
Arabic is obtained from Acacia senegal and is composed of pentose-sugar units.
Its presence as a major component in the coatings on the Raes and radiocarbon
samples is not a surprise, because it has long been a common vehicle in tempera
paints. The radiocarbon sample had been dyed. Dyeing was probably done
intentionally on pristine replacement material to match the color of the older,
sepia-colored cloth. The gum is probably the same age as the Raes and
radiocarbon yarn and should have no effect on the age determination. In any
case, this water-soluble, easily-hydrolyzed gum would have been removed
completely by the cleaning procedures used on the dated samples [1].

The dye found on the radiocarbon sample was not used in Europe before about
A.D. 1291 and was not common until more than 100 years
later [6].
The combined evidence from chemical kinetics, analytical chemistry, cotton
content, and pyrolysis/ms proves that the material from the radiocarbon area of
the shroud is significantly different from that of the main cloth. The
radiocarbon sample was thus not part of the original cloth and is invalid for
determining the age of the shroud. Because the storage conditions through the
centuries are unknown, a more accurate age determination will require new
radiocarbon analyses with several fully characterized and carefully prepared
samples.

A significant amount of charred cellulose was removed during a restoration of
the shroud in 2002 [10].
Material from different scorch locations across the shroud was saved in separate
containers. The elemental carbon could be completely cleaned in concentrated
nitric acid, thus removing all traces of foreign fibers, sebum from repeated
handling, and adsorbed thymol from an unfortunate procedure to sterilize the
shroud's reliquary in 1988. In addition, the separate samples would give a
“cluster” of dates, always a desirable procedure in archaeology. A new
radiocarbon analysis should be done on the charred material retained from the
2002 restoration.

Acknowledgements

I wish to recognize Barrie Schwortz, the record photographer for STURP, for
expert assistance in preparing the figures. None of the work reported was
supported by any part of the United States Government or any other funding
organization. All of the microscopy and wet-chemical work was done in the
author's home laboratory.